Technically, it is a source, but it is not considered reliable. WikiTree Help has the following:
Unreliable Sources
All family trees and genealogies, in all formats (websites, family genealogies published as books, CD-ROMs, family association newsletters, GEDCOMs, etc.) are secondary sources.
As discussed above, the most reliable secondary sources are ones that carefully cite primary sources. Conversely, the least reliable secondary sources are ones that do not reference any primary sources. This means that the information in them cannot be independently verified. It is second-hand information.
Family trees and genealogies may be valuable resources and may help you find original, reliable sources, but they are not reliable on their own. Family trees never qualify as reliable sources for pre-1700 profiles.